Atomic Layer Epitaxy (ALE) method was invented by Dr. Tuomo Suntola in the early 1970's. Another generic name for the method is Atomic Layer Deposition (ALD) and it is nowadays used instead of ALE. ALD is a special chemical deposition method based on the sequential introduction of at least two precursor species to a substrate that is located within a heated reaction space. The growth mechanism of ALD relies on the bond strength differences between chemical adsorption (chemisorption) and physical adsorption (physisorption). During chemisorption a strong chemical bond is formed between atom(s) of a solid phase surface and a molecule that is arriving from the gas phase. Bonding by physisorption is much weaker because only van der Waals forces are involved. Physisorption bonds are easily broken by thermal energy when the local temperature is above the condensation temperature of the molecules.
By definition the reaction space of an ALD reactor comprises all the heated surfaces that can be exposed alternately and sequentially to each of the ALD precursor used for the deposition of thin films. A basic ALD deposition cycle consists of four sequential steps: pulse A, purge A, pulse B and purge B. Pulse A typically consists of metal precursor vapor and pulse B of nitrogen or oxygen precursor vapor. Inactive gas, such as nitrogen or argon, and a vacuum pump are used for purging gaseous reaction by-products and the residual reactant molecules from the reaction space. A deposition sequence contains at least one deposition cycle. Deposition cycles are repeated until the deposition sequence has produced a thin film of desired thickness.
Precursor species form through chemisorption a chemical bond to reactive sites of the heated surfaces. Conditions are typically arranged in such a way that no more than a molecular monolayer of a solid material forms on the surfaces during one precursor pulse. The growth process is thus self-terminating or saturative. For example, the first precursor can include ligands that remain attached to the adsorbed species and saturate the surface, which prevents further chemisorption. Reaction space temperature is maintained above condensation temperatures and below thermal decomposition temperatures of the utilized precursors such that the precursor molecule species chemisorb on the substrate(s) essentially intact. This chemisorption step is typically followed by a first purge step (purge A) wherein the excess first precursor and possible reaction by-products are removed from the reaction space. Second precursor vapor is then introduced into the reaction space. Second precursor molecules typically react with the adsorbed species of the first precursor molecules, thereby forming the desired thin film material. This growth terminates once the entire amount of the adsorbed first precursor has been consumed. The excess of second precursor vapor and possible reaction by-product vapors are then removed by a second purge step (purge B). The cycle is then repeated until the film has grown to a desired thickness. Deposition cycles can also be more complex. For example, the cycles can include three or more reactant vapor pulses separated by purging steps. All these deposition cycles form a timed deposition sequence that is controlled by a logic unit or a microprocessor.
Thin films grown by ALD are dense, pinhole free and have uniform thickness. For example, aluminum oxide grown from trimethyl aluminum (CH3)3Al, also referred to as TMA, and water at 250-300° C. has usually about 1% non-uniformity over the 100-200 mm wafer. Metal oxide thin films grown by ALD are suitable for gate dielectrics, electroluminescent display insulators and capacitor dielectrics. Metal nitride thin films grown by ALD are suitable for diffusion barriers, e.g., in dual damascene structures. Precursors for the ALD growth of thin films and thin film materials deposited by the ALD method are disclosed, for example, in a review article M. Ritala et al., “Atomic Layer Deposition”, Handbook of Thin Film Materials, Volume 1: Deposition and Processing of Thin Films, Chapter 2, Academic Press, 2002, p. 103, which is incorporated herein by reference.
Apparatuses suited for the implementation of ALE and ALD methods are disclosed, for example, in review articles T. Suntola, “Atomic Layer Epitaxy”, Materials Science Reports, 4(7) 1989, Elsevier Science Publishers B.V., p. 261, and T. Suntola, “Atomic Layer Epitaxy”, Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Elsevier Science Publishers B.V., 1994, p. 601, which are incorporated herein by reference.
A batch ALD reactor, disclosed in U.S. patent application publication no. 2003/0121469 A1 that is incorporated herein by reference, contains multiple substrates and a folded gas flow path. Long flow path causes problems with processes where reaction by-products can re-adsorb on the substrate surface and block reactive surface sites.
An ALD reactor, disclosed in U.S. patent application publication no. 2003/0121608 A1 that is incorporated herein by reference, has a stationary lid with an attached gas in-feed system and a susceptor that can be lowered for accessing the substrate through a gate valve. On top of the lid there is a narrow gas mixing volume for forming uniform gas mixture before letting the gases into the substrate space.
An ALD reactor, disclosed in U.S. Pat. No. 6,660,126 that is incorporated herein by reference, has a gas pulsing system integrated with a reaction chamber lid and a susceptor that can be lowered to access the substrate through a gate valve. One of the problems with the design is that pulsing valve attached directly to the lid are rather vulnerable to gas leaks, which may result in non-uniform CVD (Chemical Vapor Deposition) growth of film on the substrate.
A single wafer ALD reactor, disclosed in U.S. patent application publication no. 2003/0150385 that is incorporated herein by reference, has a stationary lid and a susceptor (substrate holder) that can be lowered for accessing the substrate. Precursor vapors are fed to the reaction chamber from one side of the reaction chamber and gases are evacuated from the opposite side of the reaction chamber.
A CVD system, disclosed in U.S. patent application publication no. 2004/0065256 that is incorporated herein by reference, has a lid with gas conduits for reactive cleaning gas and a showerhead placed under the lid.
An ALD reactor, disclosed in U.S. patent application publication no. 2003/0183171 that is incorporated herein by reference, has a hot pre-reactor surface inside a showerhead. The substrate is accessed from a side of the reaction chamber through a gate valve. A problem with the reactor is that the expensive gate valve is exposed to the process temperature, which may drastically shorten the lifetime of the gate valve.
An ALD reactor, disclosed in U.S. patent application publication no. 2003/0180458 that is incorporated herein by reference, has a stationary lid, a gas distributor chamber and a nozzle array.
An ALD reactor, disclosed in U.S. Pat. No. 6,585,823 that is incorporated herein by reference, has a susceptor stack for processing a batch of substrates. The susceptor stack is lowered vertically from the reaction chamber so that the substrates can be accessed. Precursor vapor is fed to the reaction chamber from the bottom side of the chamber and evacuated from the top side of the chamber. A problem with the reactor is that the reaction by-products, such as hydrogen chloride HCl, can readsorb on the surfaces of the down stream side substrates causing non-uniform film growth.
An ALD reactor for coating particles, disclosed in U.S. Pat. No. 6,534,431 that is incorporated herein by reference, has a reaction chamber for coating solid particles. A problem with the reactor is that when the reactor is pressurized to atmospheric pressure and the flange of the reactor tube is opened for accessing the coated particles, any solid precursor left within the source tube is exposed to room air because the source tube is in fluid communication with the reaction chamber. It means that fresh reactive precursor must be loaded into the source tube before each deposition process.
Finally, a CVD/ALD reactor, disclosed in U.S. patent application publication no. 2004/0083960 that is incorporated herein by reference, has a reaction chamber, a lid attachable to the reaction chamber, and a connector. The connector has a first portion coupled to the lid, a second portion coupled to the reaction chamber, a gas passageway extending through the first portion and the second portion, and a seal between the first and second portions surrounding the gas passageway. The first and second portions of the connector are detachably coupled to each other.
The preceding description shows that there exist various ALD/CVD reactors with various problems. One general problem is that, in most known reactors, the top part of the reactor must laboriously be disassembled before the reaction chamber can be serviced. Also, the footprint of these reactors is rather large and they typically have high electrical power consumption.